1. Field of the Invention
The present invention relates to a method and an apparatus for producing a plastic optical element. More particularly, the present invention relates to a method and an apparatus for producing a plastic optical element involving polymerization.
2. Description of the Related Art
Plastic lenses are roughly classified into two kinds, one of which is made of thermoplastic polymers. As the thermoplastic polymers, polymethyl methacrylate (PMMA), polycarbonate, polystyrene, acrylonitrile-styrene copolymer and the like are known. These thermoplastic polymers may be molded by compression molding injection molding or injection-compression molding. In principle, for lens production, the polymer is heated and melted, poured and processed in a mold having a cavity correpsonding to the lens shape and compressed while the polymer is hot so as to compensate for a shrunk volume of the polymer which will be caused by cooling through volume elasticity. In the molding of thermoplastic polymers, since heat and pressure are necessary, large molds and apparatuses should be used, and molded articles tend to suffer from minute deformation.
Other lenses which can be produced through polymerization and processing of monomers for producing thermosetting polymers or thermoplastic polymers such as methyl methacrylate, diethylene glycol bis-allylcarbonate (namely, CR-39), glycol dimethacrylate, diethylene glycol dimethacrylate and the like. The polymers prepared from these monomers generally set at a relatively low temperature under low pressure so that they can be molded by a simple method which is referred to as casting or cast molding.
FIG. 1 illustrates one of the conventional cast molding methods for producing lenses for spectacles. A pair of glass plates 101, 102 are super posed through a gasket 103. A space formed by the the glass plates 101,102 and the gasket 103 is filled with a liquid monomer 104, and the circumferential edges of the glass plates are clamped with a clip 105. Then, the glass plates containing the monomer therein are kept standing. Since a small amount of a polymerization initiator is added to the monomer just before charging, a slight temperature increase initiates a polymerization reaction of the polymer in the space. After several to ten and several hours, the entire monomer is polymerized and forms a hard plastic lens. During polymerization, the material in the space shrinks by 10 to 20%, so that a thickness of the produced lens is decreased.
Although the cast molding can be employed to produce lenses having a relatively uniform and thin thickness such as spectacle lenses, it is said to be unsuitable for producing lenses for cameras such as VTR cameras. Further, since the cast molding takes a long time, it is not generally accepted for industrial production of plastic lenses.
To overcome the above drawbacks of the cast molding, Japanese Patent Kokai Publication No. 132221/1980 discloses, as shown in FIG. 2, a method comprising curing a resin 208 in a cavity formed with glass molds 206,207 while irradiating the resin with UV light 202, charging an additional amount of resin 201 from a reservoir 209 through a stop cock 210 into the cavity to fill a gap formed through shrinkage of the cured polymer and removing the molded article after all the resin is cured. Since the UV curing is known as a high rate reaction process, it seems that the combination of this measure with the supplement of the resin would overcome the above drawbacks. However, this method has various problems. For example, since the resin loses its flowability before the entire reaction is completed, the shrunk volume is not sufficiently supplemented. Since the entire reaction proceeds at a high rate, compression stress due to the cure and shrinkage and tensile stress due to reaction against compression stress are quickly generated so that there is no or litter time for relieving the stresses, and the stresses exceed the breaking stress of the molded article, which results in insufficient dimensional accuracy and cracking. Thus, the molded articles are often broken before they are removed from the molds. This is particularly so when the lens has a large thickness.